The overall objectives of the project are to isolate organic metabolites of selenium and to determine their toxicity and biosynthetic pathway. Work is being continued on the isolation and identification of an unknown metabolite of selenium which is found in rat urine. Upon successful identification of the substance, its toxicity and biological availability will be determined. Studies are also underway to determine whether or not a previously identified metabolite, trimethylseleninium ion, is a product of selenium metabolism in species other than the rat. As time permits, studies will be conducted to elucidate the mechanism by which arsenic potentiates the toxicities of various selenium excretory products and other methylated selenium derivatives.